1. Field of the Invention
The invention concerns an improved single-mode, polarization-maintaining optical fiber, especially one that is polarization-maintaining by virtue of an oval or elliptical stress-applying region. The preservation of polarization is especially important in sensor fibers such as are used in fiber gyroscopes and interferometric sensors.
2. Description of the Related Art
A single-mode optical fiber typically has a glass inner core of high index of refraction, a cladding of low index of refraction, and a silica jacket surrounding the cladding. The diameter of the core is from 3 to 10 micrometers, and the diameter of the jacket is 80 micrometers for sensor fibers and 125 micrometers for telecommunications. According to Shibata et al.: "Fabrication of Polarization-Maintaining and Absorption-Reducing Fibers," Journal of Lightwave Technology, Vol. LT-1, No. 1, pp. 38-43 (1983) at page 38:
"The general approach to maintaining linear polarization in single-mode fibers is to increase fiber birefringence so as to reduce the power interchange between polarization modes. Several kinds of highly birefringent single-mode fibers have been demonstrated: fibers with a noncircular core, which cause birefringence due to noncircular geometry [citing Ramaswamy et al.: `Polarization Characteristics of Noncircular Core Single-mode Fibers`, Applied Optics, Vol. 17, No. 18, pp 3014-3017 (1978)]; fibers with an elliptical cladding, which cause anisotropic strains in the core [citing Ramaswamy et al.: `Birefringence in Elliptically Clad Borosilicate Single-mode Fibers`, Applied Optics, Vol. 18, No. 24, pp 4080-4084 (1979) and Katsuyama et al.: `Low-loss Single Polarization Fibers,` Electron. Lett., Vol. 17, No. 13, pp 437-474 (1981)]; and fibers with refractive-index pits on both sides of the core [citing Hosaka et al.: `Single-mode Fiber with Asymmetrical Refractive Index Pits on Both Sides of Core,` Electron. Lett., Vol. 17, No. 5, pp 191-193 (1981)]." PA0 "were fabricated with jacketing techniques. A single-mode fiber preform made by the VAD method [citing Tomaru et al.: `Fabrication of Single-mode Fibers by VAD,` Electronics Lett., Vol. 16, No. 13, pp 511-512 (1980)] was elongated to several millimeters in diameter. Then, it was put into the center of a thick-wall jacketing silica tube with about 15-mm inner diameter. Stress-applying parts were prepared by a depositing SiO.sub.2 -B.sub.2 O.sub.3 -GeO.sub.2 glass layer in a silica tube via the MCVD method. The rods prepared by the MCVD method were also elongated to several millimeters and arranged on both sides of the core rod in the jacketing tube. The remaining inner spaces in the jacketing tube were filled with several commercially available silica rods, for example, four rods with several millimeters of diameter. The final preforms were drawn into fibers by a carbon-resistance furnace."
The Shibata publication concerns a birefringent single-mode fiber, the silica jacket of which encloses two fan-shaped, diametrically-opposed regions that have been doped to have a different thermal expansion than does the rest of the jacket, resulting in anisotropic stress-induced birefringence in the core. At page 40, the Shibata publication says these fibers
A birefringent single-mode fiber having similarly-shaped stress-applying regions as well as an elliptical core is shown in U.S. Pat. No. 4,480,897 (Okamoto et al.). FIGS. 6, 8, 11 and 12 of U.S. Pat. No. 4,561,871 (Berkey) also illustrate the manufacture of birefringent single-mode fibers having diametrically-opposed stress-applying regions separated from the core.
Katsuyama et al.: "Low-loss Single Polarization Fibers," Applied Optics, Vol. 22, No. 11, pp 1741-1747, 1983, concerns a single-mode, polarization-maintaining optical fiber having three concentric silica regions that make up the cladding. The intermediate region (which the Katsuyama publication calls the "elliptical-jacket" or simply "jacket") is made stress-applying by being doped with B.sub.2 O.sub.3. This intermediate stress-applying region also is doped with GeO.sub.2 in order to make its refractive index the same as that of the pure silica in the inner and outer barrier regions.
A procedure for making a single-mode, polarization-maintaining optical fiber that has an elliptical stress-applying region is disclosed in U.S. Pat. No. 4,274,854 (Pleibel et al.). After grinding a hollow substrate tube of quartz (pure silica) to have two diametrically opposed flat surfaces, a series of siliceous layers are deposited onto the interior surface of the substrate tube, after which the tube is collapsed to provide a preform and then drawn into a fiber. Because the material of the substrate tube cools first, its inner surface is elliptical in cross section and constrains the deposited siliceous layers so that they subject the core to an asymmetric stress, giving rise to birefringence. Although not disclosed in the Pleibel patent, birefringent single-mode optical fibers now on the market that have an elliptical stress-applying region also have an inner barrier of substantially circular cross section between the core and the stress-applying region. As pointed out in the Katsuyama publication, the inner barrier minimizes absorption or light transmission losses. See also Cohen et al.: "Radiating Leaking-Mode Losses in Single-Mode Lightguides with Depressed-Index Claddings", IEEE J. of Quantum Elec., QE-18, p. 1467 (1982).
While the above citations teach methods for making a single-mode fiber birefringent, none of them considers the adverse effects of either macro-bending or micro-bending, both of which are of great importance for applications wherein the fiber is coiled into small packages, e.g., in gyroscopes. It is known that both macro-bending and micro-bending result in signal attenuation and that this result can be minimized by increasing the refractive index difference between the core and cladding to reduce the mode-field diameter. See Isser et al.: "Bending and Microbending Performance of Single-Mode Fibers," Technical Report TR-63, January 1987, from Corning Glass Works which describes tests for such losses on fibers that are not fully identified. This increased refractive index difference has been accomplished by increasing the core germanium oxide concentration, but the resulting improvement in bending performance then comes at the expense of higher transmission losses. See Ainslie et al.: "Interplay of Design Parameters and Fabrication Conditions on the Performance of Monomode Fibers Made by MCVD," IEEE, Vol. QE-17, No. 6, pp 854-857 (1981), FIG. 1 of which shows these losses in fibers that are not polarization-maintaining. That these losses apply to polarization-maintaining fibers is shown in Rashliegh et al.: "Polarisation Holding in Coiled High-Birefringence Fibers", Elec. Ltrs., Vol. 19, No. 20, pp. 850-851 (1983).
A problem encountered when the core of an optical fiber is highly doped with germanium oxide is that the single-mode optical fiber may be degraded by exposure to ionizing radiation such as is commonly encountered in satellites as well as in many other locations. See Brambani et al.: "Radiation Effects in Polarization-maintaining Fibers," Paper No. 992-07, SPIE Internatioal Symposium on Fiber Optics, Optoelectronics and Laser Applications, Boston, Mass, September, 1988.